The DeleteOnRelease setting is new in V1.9.
[Settings] DeleteOnRelease=0 ; Default: 1
It defines whether a client entry in the databse is deleted upon DHCPRELEASE, or not. The default behavior is to automatically delete the entry in the database. The effect is that when the same client comes back after some time and asks for an IP address, no information about it’s previous IP address is available. Even if the old IP address is available, it would most likely not being chosen. With DeleteOnRelease set to 0, the entry only gets an expired lease timestamp and can therefore be reactivated at a later time.
In addition to this the DeleteOnRelease setting also controls the bahvior for expired leases. When DeleteOnRelease is set then expired leases are also automatically deleted from the INI file same as DHCPRELEASE from client. This is new in V1.9.3 and is based on a timeout mechansim. The timeout is set to the time when the next client lease expires. This is updated every time a DHCP action is happening.